Passive optical networks are low cost access networks that provide elements to deliver telecommunication signals from a central office via an Optical Line Terminal OLT to subscribers near Optical Network Units ONU. The direction from the central office to the subscribers is called downstream while the reverse direction is called upstream. Since a single OLT is connected to several ONUs through a complete passive optical infrastructure, the downstream communication is of the broadcast type, which means that all ONUs connected to the OLT receive the same signal emitted by the OLT. Upstream communication is of the point to point type, as all signals from the ONUs are merely combined together in the splitters. This requires channel sharing, which is usually done by a time division multiplexing mechanism.
Under normal circumstances, an ONU transmits data during a previously assigned time slot. Correct decoding of signals can be compromised when two or more signals collide and arrive superimposed at the OLT. Such a situation can occur when there is a failure in the ONU and it starts transmission of data outside the assigned time slot. This can leave the entire optical network under a severe denial of service condition.
Until now once a faulty ONU was identified, a technician would have been sent to its location, in order to physically disconnect it. During this procedure the network remain unavailable, leading to network downtime.
In the European Patent EP 1 376 918 B1 a passive optical network is disclosed, which uses auxiliary optical lines and where a bistable optical switch in the splitter connects to the auxiliary optical line, if the signal on the main optical line is faulty or the optical line is broken. The electronic circuitry for controlling the bistable switch is powered by an optical signal, which has a wavelength outside the range for the use by the telecommunication signals. The bistable switch is activated, if the signal on the main optical line is below a predefined threshold.